Compressors, such as centrifugal compressors, are widely used in extraction applications, processing plants and pipeline applications to compress and distribute gas streams. A common arrangement of a known compression system 100 for such applications is shown in FIG. 1, where typical components are visible.
The compression system 100 comprises a station inlet header 101 for receiving a medium and a station outlet header 102 for providing the medium after having been processed by the compression system 100. Between the station inlet header 101 and the station outlet header 102 there are provided two centrifugal compressors 103,104. The centrifugal compressor 104 is controlled by a variable frequency drive 105 and motor 106. From the station inlet header 101 the medium passes process and safety valves, one of which is referenced at 107, and scrubbers 108,109 before being fed to the centrifugal compressors 103,104. The medium compressed by the centrifugal gas compressor 103 is partially used as fuel gas in the gas turbine 110 to drive the gas compressor 110. On its way from the centrifugal compressors 103,104 the medium passes gas coolers 111 and 112.
Due to external as well as internal disturbances and interactions between different control layers, the components of the control system can suffer from oscillations during normal operation. This is generally undesired as the stability of the process is disturbed and the piping system and other components of the control system are put under stress.
The responsibility of control of centrifugal compressors is commonly shared by an anti-surge controller and a process controller. The anti-surge controller is responsible to keep the compression system in the stable operating region on the right side of the surge line, whereas the process controller regulates either the discharge flow, the suction pressure, the discharge pressure or the pressure ratio to the set point requirements coming from higher automation levels or human operators by manipulating the speed or torque of the driving system. Other control systems might be present depending on specific applications, e.g. a suction pressure controller manipulating an upstream valve. In the current industry practice these controllers are all using distributed proportional-integral-derivative (PID) loops which are typically not communicating with each other.
During normal operation it is often the case that small oscillations are present in the compression system. These oscillations can be caused by a number of effects. Some of these effects will be summarized next.
A mismatch between set point and real value written to the actuator (especially important for valves, which have usually inaccurate actuation/positioning systems) may cause oscillations.
Stick-Slip behavior of recycle valves and other process valves may cause oscillations. This is disclosed in “Modelling Valve Stiction” by M. A. A. Shoukat Choudhury, N. F. Thornhill and S. L. Shah, Control Engineering Practice 13 (2005) 641-658, 2005.
Persistent or pulsating disturbances upstream and downstream of the considered application may cause oscillations.
Wet gas conditions may cause oscillations.
Not considered interactions between different control loops (e.g. two process controllers of two different machines oscillate against each other) may cause oscillations.
Badly tuned controllers in the compression system (e.g. the process controller) or outdated tuning (e.g. process conditions changed significantly) may cause oscillations.
Such oscillations are not desired due to the fact that they lower the lifetime of components and perturb the stability of the control system and the quality of the control performance.
Dead-time compensation and/or detuning of the control loops help to deal with oscillations at the cost of reduced control performance. Such mechanisms are passive measures that can help or improve process damping.
However, there is still a need for an improved damping of oscillations in a control process.